fix: enrich route destination names from airport DB when not stored

Specific-airports mode scans never resolved full airport names — they
stored the IATA code as destination_name. Fixed in two places:

- airports.py: add lookup_airport(iata) cached helper
- api_server.py: enrich destination_name/city on the fly in the routes
  endpoint when the stored value equals the IATA code (fixes all past scans)
- scan_processor.py: resolve airport names at scan time in specific-airports
  mode using lookup_airport (fixes future scans at the DB level)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

flight-comparator/scan_processor.py

@@ -15,7 +15,7 @@ from datetime import datetime, date, timedelta
 import json
 
 from database import get_connection
-from airports import get_airports_for_country
+from airports import get_airports_for_country, lookup_airport
 from searcher_v3 import search_multiple_routes
 
 
@@ -180,7 +180,10 @@ async def process_scan(scan_id: int):
             else:
                 # Specific airports mode: parse comma-separated list
                 destination_codes = [code.strip() for code in country_or_airports.split(',')]
-                destinations = []  # No pre-fetched airport details; fallback to IATA code as name
+                destinations = [
+                    lookup_airport(code) or {'iata': code, 'name': code, 'city': ''}
+                    for code in destination_codes
+                ]
                 logger.info(f"[Scan {scan_id}] Mode: Specific airports ({len(destination_codes)} destinations: {destination_codes})")
 
         except Exception as e: